improve JAWS on FF's handling of file search form
improve JAWS on FF's handling of file search form

fixes ADMIN-933

test plan:
 - with JAWS+Firefox, go to Course->Files
 - ensure 'Search for files' is read when
   using 'f' and up/down to navigate forms

Change-Id: I19664f340119e8fae45574ecaf096ef7efdffbaf
Reviewed-on: https://gerrit.instructure.com/148317
Tested-by: Jenkins
Reviewed-by: Mysti Sadler <431d062aff1e8f25416b3a7b732fe6effdb03f9a@instructure.com>
QA-Review: Dan Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>
Product-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms